Reference for Shabbat 9:2
מִנַּיִן לִסְפִינָה שֶׁהִיא טְהוֹרָה, שֶׁנֶּאֱמַר (משלי ל) דֶּרֶךְ אֳנִיָּה בְלֶב יָם. מִנַּיִן לַעֲרוּגָה שֶׁהִיא שִׁשָּׁה עַל שִׁשָּׁה טְפָחִים שֶׁזּוֹרְעִין בְּתוֹכָהּ חֲמִשָּׁה זֵרְעוֹנִין, אַרְבָּעָה בְאַרְבַּע רוּחוֹת הָעֲרוּגָה וְאֶחָד בָּאֶמְצַע, שֶׁנֶּאֱמַר (ישעיה סא) כִּי כָאָרֶץ תּוֹצִיא צִמְחָהּ וּכְגַנָּה זֵרוּעֶיהָ תַצְמִיחַ, זַרְעָהּ לֹא נֶאֱמַר, אֶלָּא זֵרוּעֶיהָ:
Whence is it derived that a ship is clean [i.e., that it does not acquire uncleanliness]? From (Proverbs 30:19): "the way of a ship in the heart of the sea." [Is it not obvious that a ship is in the heart of the sea? (i.e., What are we being taught by this?) We are being apprised that the ship is like the sea, i.e., just as the sea is clean, so a ship is clean — even one of earthenware; even one that was laden on dry land and lowered into the sea.] Whence is it derived that in a garden bed six by six cubits, five (different varieties of) seeds may be sown? Four on the four sides of the bed, and one in the middle? From (Isaiah 61:11): "For as the earth brings forth her growth, and as a garden sprouts its seeds." It is not written "its seed," but "its seeds." [("five (different varieties of) seeds":) so that there be enough separation between them that they not constitute a (forbidden) admixture (kilayim). ("Four on the four sides of the bed":) He fills every side close to the corner, and in the middle he sows only one kernel, so that it is three cubits distant from what is sown on every side. For the "nurturing" distance between one seed and another is a cubit and a half. And even though near the corners in each direction the seeds are close to each other, not being three cubits removed, and being nourished by one another, there is no reason for apprehension here, for it was kilayim (the forbidden admixture) itself that Scripture was concerned with, and not with the nourishment factor. As it was taught: If there were a fence between them, one can sow next to the fence on one side; and the other, next to the fence on the other, even though they are mutually nourished from beneath. And here (in our instance) there is a clear separation; for one side is sown north-south and the other, east-west. But between the middle seed to those at the sides, there is no clear separation, and if they were close, they would constitute an admixture, for which reason they require distancing to the extent of nourishing. ("For as the earth brings forth her growth, and as a garden sprouts its seeds":) "brings forth" — one; "her growth" — one; "its seeds" — two; "sprouts" — one; all together — five. (That it be in an area of six cubits by) six cubits is not derived from the verse; but the rabbis know that with five varieties of seed in such an area, the corners are not nourished by the middle nor the middle by the corners, the nourishment distance of each seed being a cubit and a half. So that when Scripture intimates five seeds, it is understood to be in an area of six by six cubits. And in Tractate Kilyaim I have explained the halachoth of garden beds in detail. Here I have been brief.]